1. Field of the Invention
This invention relates to computer systems and, more particularly, to methods for varying the amount of power used by such systems during use of the systems.
2. History of the Prior Art
A significant problem faced by battery powered computers is the length of time such computers are capable of operating between charges. As computer processors become more capable, they tend to run at faster speeds and dissipate more power. At the same time, the size and weight of portable computers is constantly being reduced to make them more portable. Since batteries tend to be a very significant element of the weight of portable computers and other portable devices, the tendency has been to maintain their size and thus their capacity at a minimum.
A typical portable computer today has an average life of approximately two and one-half hours until its originally-full battery must be discharged.
A great deal of research has been directed to ways for extending the operating life of portable computers. Presently, typical processors include circuitry and software for disabling various power-draining functions of portable computers when those functions are unused for some extensive period. For example, various techniques have been devised for turning off the screen when it has not been used for some selected period. Similar processes measure the length of time between use of hard drives and disable rotation after some period. Another of these processes is adapted to put a central processor into a quiescent condition after some period of inactivity.
In general, these processes are useful in extending the operating life of a portable computer. However, the life still does not extend significantly beyond two and one-half hours for any computer having significant capabilities.
There has been a significant amount of research conducted from which processor requiring less power might be produced. Most processors used in computer systems today are made using CMOS technology. The power consumed by a CMOS integrated circuit is given approximately by P=CV2f, where C is the active switching capacitance, V is the supply voltage, and f is the frequency of operation. The maximum allowable frequency is described by fmax=kV, where k is a constant.
It is desirable to operate the processor at the lowest possible voltage at a frequency that provides the computing power desired by the user at any given moment. For instance, if the processor is operating at 600 MHz, and the user suddenly runs a compute-intensive process half as demanding, the frequency can be dropped by a factor of two. This means that the voltage can also be dropped by a factor of two. Therefore, power consumption is reduced by a factor of eight. Various methods of implementing this dynamic voltage-frequency scaling have been described in the prior art. All of these involve a component separate from the processor on the system that provides multiple frequencies to multiple system components. Also, they involve state-machines or power-management units on the system to coordinate the voltage-frequency changes. The efficiency of voltage frequency scaling is reduced when the frequency generator is not on the processor. Having a separate power-management unit increases the number of components in the system and the power dissipated by the system. It is also desirable to have the processor control both the voltage it receives and the frequency it receives. As the level of integration increases in processors, they control most of the system clocks; and it is desirable to provide control to the processor to change these clocks so they can be run at just the right frequency. Having a separate clock generator that produces multiple frequencies is not desirable because of the lack of tight coupling.
It is desirable to increase significantly the operating life of portable computers and similar devices.